Field Test of An X-Ray Sediment-Concentration Gage

A suspended sediment concentration gauge was field tested in flood flows ranging to 5,000 cfs. with sediment concentrations up to 14,000 ppm. The nuclear gage, powered by storage batteries, has a sensing unit in the flow and a recording unit on shore, connected by a water-tight flexible cable. The magnitude of attenuation of X-rays, provided by a cadmium-109 source (about 2 millicuries, and 470 days half-life), was a measure of sediment concentration. A switching mechanism caused the beam to alternately pass through distilled water and the ambient water of the creek. The time of counting was sufficient to receive a predetermined number of pulses through the distilled water reference cell. The gage was found to be sensitive not only to the suspended sediment but also to air temperature and to instrument age. The standard deviation of the 15-min values was about 200 ppm. Gage readings, corrected for age and air temperature, gave concentrations comparable to those determined by conventional sampling of the flood waters.